Diabetes mellitus is a major health problem in the United States and throughout the world's developed and developing nations. In 2002, the American Diabetes Association (ADA) estimated that 18.2 million Americans—fully 6.4% of the citizenry—were afflicted with some form of diabetes. Of these, 90–95% suffered from type 2 diabetes, and 35%, or about 6 million individuals, were undiagnosed. See ADA Report, Diabetes Care, 2003. The World Health Organization (WHO) estimates that 175 million people worldwide suffer from diabetes; type 2 diabetes also represents 90% of all diagnoses worldwide. Unfortunately, projections indicate that this grim situation will worsen in the next two decades. The WHO forecasts that the total number of diabetics will double before the year 2025. Similarly, the ADA estimates that by 2020, 8.0% of the U.S. population, some 25 million individuals, will have contracted the disease. Assuming rates of detection remain static, this portends that, in less than twenty years, three of every 100 Americans will be ‘silent’ diabetics. It is no surprise that many have characterized the worldwide outbreak of diabetes as epidemic.
Diabetes has a significant impact on individual health and the national economy. U.S. health care costs related to diabetes exceeded $132 billion in 2002. Due to the numerous complications that result from chronic hyperglycemia, these costs were distributed over a wide array of health services. For example, between 5 and 10 percent of all U.S. expenditures in the areas of cardiovascular disease, kidney disease, endocrine and metabolic complications, and ophthalmic disorders were attributable to diabetes. See ADA Report, Diabetes Care, 2003. These economic and health burdens belie the fact that most diabetes-related complications are preventable. The landmark Diabetes Control and Complications Trial (DCCT) established that a strict regimen of glucose monitoring, exercise, proper diet, and insulin therapy significantly reduced the progression of and risk for developing diabetic complications. See DCCT Research Group, N Eng J Med, 1993. Furthermore, the ongoing Diabetes Prevention Program (DPP) has already demonstrated that individuals at risk for diabetes can significantly reduce their chances of contracting the disease by implementing lifestyle changes such a weight loss and increased physical activity. See DPP Research Group, N Eng J Med, 2002. ADA has recommended that health care providers begin screening of individuals with one or more disease risk factors, observing: “If the DPP demonstrates a reduction in the incidence of type 2 diabetes as a result of one or more of the [tested] interventions, then more widespread screening . . . may be justified”. See ADA Position Statement, Diabetes Care, 2003.
The Fasting Plasma Glucose (FPG) test is one of two accepted clinical standards for the diagnosis of or screening for diabetes. See ADA Committee Report, Diabetes Care, 2003. The FPG test is a carbohydrate metabolism test that measures plasma glucose levels after a 12–14 hour fast. Fasting stimulates the release of the hormone glucagon, which in turn raises plasma glucose levels. In non-diabetic individuals, the body will produce and process insulin to counteract the rise in glucose levels. In diabetic individuals, plasma glucose levels remain elevated. The ADA recommends that the FPG test be administered in the morning because afternoon tests tend to produce lower readings. In most healthy individuals, FPG levels will fall between 70 and 100 mg/dl. Medications, exercise, and recent illnesses can impact the results of this test, so an appropriate medical history should be taken before it is performed. FPG levels of 126 mg/dl or higher indicate a need for a subsequent retest. If the same levels are reached during the retest, a diagnosis of diabetes mellitus is typically rendered. Results that measure only slightly above the normal range may require further testing, including the Oral Glucose Tolerance Test (OGTT) or a postprandial plasma glucose test, to confirm a diabetes diagnosis. Other conditions which can cause an elevated result include pancreatitis, Cushing's syndrome, liver or kidney disease, eclampsia, and other acute illnesses such as sepsis or myocardial infarction.
Because it is easier to perform and more convenient for patients, the FPG test is strongly recommended by the ADA and is in more widespread use than the other accepted diagnostic standard, the OGTT. The OGTT is the clinical gold standard for diagnosis of diabetes despite various drawbacks. After presenting in a fasting state, the patient is administered an oral dose of glucose solution (75 to 100 grams of dextrose) which typically causes blood glucose levels to rise in the first hour and return to baseline within three hours as the body produces insulin to normalize glucose levels. Blood glucose levels may be measured four to five times over a 3-hour OGTT administration. On average, levels typically peak at 160–180 mg/dl from 30 minutes to 1 hour after administration of the oral glucose dose, and then return to fasting levels of 140 mg/dl or less within two to three hours. Factors such as age, weight, and race can influence results, as can recent illnesses and certain medications. For example, older individuals will have an upper limit increase of 1 mg/dl in glucose tolerance for every year over age 50. Current ADA guidelines dictate a diagnosis of diabetes if the two-hour post-load blood glucose value is greater than 200 mg/dl on two separate OGTTs administered on different days.
In addition to these diagnostic criteria, the ADA also recognizes two ‘pre-diabetic’ conditions reflecting deviations from euglycemia that, while abnormal, are considered insufficient to merit a diagnosis of diabetes mellitus. An individual is said to have ‘Impaired Fasting Glucose’ (IFG) when a single FPG test falls between 100 and 126 mg/dl. Similarly, when the OGTT yields 2-hour post-load glucose values between 140 and 200 mg/dl, a diagnosis of ‘Impaired Glucose Tolerance’ (IGT) is typically rendered. Both of these conditions are considered risk factors for diabetes, and IFG/IGT were used as entrance criteria in the Diabetes Prevention Program. IFG/IGT are also associated with increased risk of cardiovascular disease.
The need for pre-test fasting, invasive blood draws, and repeat testing on multiple days combine to make the OGTT and FPG tests inconvenient for the patient and expensive to administer. In addition, the diagnostic accuracy of these tests leaves significant room for improvement. See, e.g., M. P. Stern, et al., Ann Intern Med, 2002, and J. S. Yudkin et al., BMJ, 1990. Various attempts have been made in the past to avoid the disadvantages of the FPG and OGTT in diabetes screening. For example, risk assessments based on patient history and paper-and-pencil tests have been attempted, but such techniques have typically resulted in lackluster diagnostic accuracy. In addition, the use of glycated hemoglobin (HbA1c) has been suggested for diabetes screening. However, because HbA1c is an indicator of average glycemia over a period of several weeks, its inherent variability combines with the experimental uncertainty associated with currently-available HbA1c assays to make it a rather poor indicator of diabetes. See ADA Committee Report, Diabetes Care, 2003. HbA1c levels of diabetics can overlap those of nondiabetics, making HbA1c problematic as a screening test. A reliable, convenient, and cost-effective means to screen for diabetes mellitus is needed. Also, a reliable, convenient, and cost-effective means for measuring effects of diabetes could help in treating the disease and avoiding complications from the disease.
U.S. Pat. No. 5,582,168 (Samuels) discloses apparatus and methods for measuring characteristics of biological tissues and similar materials. These apparatus and methods are described with respect to measurements of the human eye. In addition, the correction methodologies described by these inventors involve only measurements of the elastically scattered excitation light. Samuels describes a simple linear correction technique. Samuels does not disclose an algorithm or methods by which tissue disease status may be discriminated via noninvasive measurements.
U.S. Pat. No. 6,505,059 (Kollias) discloses instruments and methods for noninvasive tissue glucose level monitoring. Kollias does not describe any method by which measured fluorescence can be corrected for the effects of tissue absorption and scattering. While Kollias indicates that a tissue reflectance measurement can be made to measure tissue scattering directly, it does not indicate how one would use this information to obtain information regarding the tissue fluorescence spectrum. Furthermore, Kollias does not disclose an algorithm or methods by which tissue disease status may be determined from noninvasive measurements.
U.S. Pat. No. 6,571,118 (Utzinger) discloses methods and apparatus for performing fluorescence and spatially resolved reflectance spectroscopy on a sample. While Utzinger describes a technique in which a combination of fluorescence and reflectance measurements are used to characterize biological tissue, the application does not relate to spectroscopy of the skin. Furthermore, the reflectance measurements described in Utzinger are spatially-resolved in nature, that is, the reflectance spectroscopy is to be conducted at one or more specific source-receiver separations. Finally, no algorithm or process is described by which the measured fluorescence may be corrected using the tissue reflectance measurements to obtain or approximate the intrinsic fluorescence spectrum of the tissue in question.
U.S. Patent application 20030013973 (Georgakoudi) discloses a system and methods of fluorescence, reflectance and light scattering spectroscopy for measuring tissue characteristics. Georgakoudi discusses estimation of intrinsic fluorescence using reflectance properties as applied to detection of esophageal cancer and Barrett's esophagus. Georgakoudi does not describe any specific techniques for such estimation.
U.S. Pat. No. 6,088,606 (Ignotz) discloses a system and method for determining the duration of a medical condition. Ignotz mentions fluorescence, but does not use a reflectance spectrum to obtain or estimate an intrinsic fluorescence spectrum. In addition, Ignotz described methods relating to determining the duration of a disease, not for diagnosing or screening for the presence of disease or for quantifying the concentration of specified chemical analytes. Finally, Ignotz does not address skin as a useful measurement site.
U.S. Pat. No. 5,601,079 (Wong) describes an apparatus for the non-invasive quantification of glucose control, aging, and advanced Maillard products by stimulated fluorescence. Wong specifically quantifies Advanced Glycation Endproducts in the blood, not in the skin and/or its structural proteins. In addition, the fluorescence correction methodology involves only measurements of the elastically scattered excitation light. Wong describes only a simple linear correction technique. Finally, Wong does not disclose an algorithm or methods by which tissue disease status may be discriminated via noninvasive measurements.
International patent publication WO 01/22869 (Smits) describes an apparatus for non-invasive determination of skin autofluorescence. The apparatus consists of a broadband uv source (blacklight) that illuminates skin through interchangeable optical bandpass filters. Resulting skin fluorescence is fiber-optically coupled to a compact spectrophotometer. The application proffers AGE concentration in the skin can be inferred from qualitative assessment of skin autofluorescence but it does not describe any means by which the AGE content can be quantified using the device and measurement techniques. The apparatus is intended to assess skin fluorescence in healthy individuals and does not address the utility of the device for disease determination. The application notes that individual skin coloring and substructure can be a measurement interferent but it is silent on techniques or methods to compensate for these variable characteristics.